Axle drives of the type which transfer torque in motor vehicles by means of a double coupling are known from DE 40 21 747 C2. Actuation takes place by a single, shared actuating device which uniformly loads both friction couplings and which must be provided with an external energy supply unit and an external controlling device. It is possible for the double coupling to be controlled independently of the various operating parameters, but it is then necessary to provide additional means for controlling the self-locking mechanical differential drives, with the cost of such means not always being justified.
An axle drive of a similar type is known from DE 38 21 773 A1 which, in contrast to the above-described drive, is provided with separate external actuating devices for the two friction couplings. In this case, too, control can be independent of the different operating parameters, with the control means providing an additional, improved individual locking effect for the two friction couplings. However, the expenditure (in space, weight, and the like) for energy supply means and control means is doubled.
Similarly-designed assemblies known as Twin Visco-Lok(copyright) couplings differ in that the friction couplings are replaced by prior art viscous couplings which are arranged in a common housing and, for each of the two axle shafts, a locking effect relative to the coupling housing is built up automatically in a differential speed sensing way. Such assemblies are simplified in their design as compared to the above-mentioned axle drives and they have the advantage of a differential-speed-dependent locking effect, but as compared to assemblies with mechanical friction couplings, they have a relatively low performance density (the performance/space ratio of the device, i.e., the ratio of the performance of the device to the space occupied by the device) and the disadvantage of independent locking effects at the two axle shafts.
An axle drive of the first-mentioned type is known from DE 44 44 027 C2 wherein two friction coupling units are coupled to the coupling carrier on the one hand and to the two axle shafts on the other hand while being loaded by a Visco-Lok(copyright) actuating device, there being a relative speed between an axle shaft and the coupling carrier.
As far as installation space, weight and costs are concerned, the latter Twin Visco-Lok(copyright) coupling constitutes an advantageous system in a hang-on configuration wherein the wheels of an axle which is dragged along can be driven if required. A hang-on configuration or system is one in which a four-wheel drive vehicle""s rear axle is driven via a coupling which closes only when there is wheel spin at the front axle, i.e., when a speed differential exists between the coupling""s input and output shafts. Such an assembly can replace a locking central differential and a locking axle differential.
With a Twin Visco-Lok(copyright) coupling which combines the longitudinal and transverse locking effect, it becomes more complicated to adjust the system with respect to traction and braking stability, and there is a tendency of driveline torsion when the vehicle is driven in a circle.
In the above-mentioned publication DE 44 44 027 C2, there is a hydraulic connection between the two actuating devices. A connection between the pressure chambers is proposed, but is disadvantageous in that, if there are different differential speeds of the two axle shafts relative to the coupling carrier, the locking moment for both axle shafts is defined only by the pump with the smaller relative speed, preventing the traction potential of the wheel of the dragged-along axle with the higher friction value to be fully utilized.
It is therefore the object of the present invention to provide an improved process of controlling an axle drive of said type and the respective axle drive which more satisfactorily meets the requirements for a hang-on system in different driving situations.
The present invention is carried out by and embodied in a method and apparatus for distributing torque from a propulsion shaft to two axle shafts. A coupling carrier is driveable by the propulsion shaft and rotatably supported in an axle drive housing, and two controllable friction couplings are mounted in the coupling carrier for selectively connecting and driving the axle shafts, with first friction plates of each friction coupling being non-rotatably connected to the coupling carrier and with second friction plates of each friction coupling being non-rotatably connected to an associated axle, with actuating devices each comprising an axially-displaceable piston acting on the friction couplings, and pump assemblies which, as a function of the relative rotation between the coupling carrier and the respective axle shaft, build up a fluid pressure which acts on the respective pistons. The objective of the present invention is achieved by providing a process and device in which, when the two axle shafts both rotate in the same direction relative to the coupling carrier, the two pump assemblies function autonomously (independently of one another), and when the two axle shafts rotate in opposite directions relative to the coupling carrier, the two pump assemblies are shut down. The two pump assemblies may be shut down by cross-wise connecting their suction and pressure ends. Also, the suction ends of the two pump assemblies can be connected to a common reservoir when functioning autonomously.
Such a system meets almost all essential requirements, i.e. maximum traction, no driveline torsion when driving the vehicle in a circle, but at the same time exerting a minimum influence on the braking stability in the case of different slip conditions on the two sides of the vehicle or between the different axles of the vehicle.
Although the above-mentioned principles apply to any pump assembly, the preferred embodiment refers to Visco-Lok(copyright) pump assemblies which are characterized in that, for each pump assembly, there is provided a pump housing in the coupling carrier; that one piston each is axially displaceable within the pump housings and delimits a pressure chamber at one end; that the pressure chambers are each connected to a reservoir and that the reservoirs and the pressure chambers are filled with a highly viscous fluid; that in each pressure chamber, there is provided a shear-channel-and-control element which, relative to the pump housing, is rotationally moveable between two end positions; and that in each pressure chamber, there is provided one shear plate, each of which is non-rotatably connected to one of the axle shafts; that rotational faces of the shear plate and counter faces of the shear-channel-and-control element form at least one closed shear channel, which closed shear channel is formed by a circumferential groove extending between two ends in the circumferential direction in the shear-channel-and-control element and by a surface of the shear plate covering said groove; that, via channels and grooves arranged in the pump housing and via control apertures provided at the ends of the circumferential grooves in the shear-channel-and-control element, the reservoir can be connected to the pressure chambers in such a way that, in both end positions of a shear-channel-and-control element, the control aperture arranged at the front end of the circumferential groove in the relative direction of rotation communicates with the reservoir; and that the control aperture positioned at the rear end of the circumferential groove in the relative direction of rotation communicates with the pressure chamber, so that, in the case of a relative speed between an axle shaft and the coupling carrier, fluid is conveyed in the shear channel from the reservoir to the pressure chamber. In particular, it is proposed that the coupling carrier forms a common pump housing for both pump assemblies, which are designed symmetrically relative to a common intermediate housing wall, and that the two shear-channel-and-reversing elements rest against the intermediate housing wall. An advantageous inventive design solution consists in that the intermediate housing wall is provided with two radial surface grooves which are positioned back-to-back and which connect the pressure end of the respective shear channel to the respective pressure chamber, and furthermore with two radial inner channels which are positioned at the same circumferential angular distance from the surface grooves and which end in axial through apertures connecting the suction end of the respective shear channel to the reservoir, and finally with two further through-apertures arranged at the same circumferential distance from each of the above-mentioned through-apertures; that the shear-channel-and-control elements, on the side resting against the intermediate housing wall, are provided with a radial surface groove which, in one of the end positions, connects one of the outer additional through-apertures to the pressure chamber; and that the control apertures in the shear-channel-and-reversing elements, in the circumferential direction, simultaneously cover at least one of the through-apertures and one of the additional through apertures in the intermediate housing wall.
The above-mentioned arrangement of cross-wise connections in the intermediate housing wall has a shut-down function when the two axle shafts rotate relative to the coupling carrier in different directions, thereby effectively keeping the connections closed, with each pump functioning autonomously when both axle shafts rotate in the same direction relative to the coupling carrier.